Power factor is the ratio of real power to apparent power. In the United States, power is provided at approximately 120 Volts AC with a frequency of approximately 60 Hertz. In Europe and other areas, power is provided at approximately 240 Volts AC with a frequency of approximately 50 Hertz.
Power factor correction (PFC) is the process of adjusting the characteristics of electric loads that create a power factor less than 1. Power factor correction may be applied either by an electrical power transmission utility to improve the stability and efficiency of the transmission network. Or, power factor correction may be installed by individual electrical customers to reduce the costs charged to them by their electricity supplier. A high power factor (i.e., close to unity, or “1”) is generally desirable in a transmission system to reduce transmission losses and improve voltage regulation at the load.
Electrical loads consuming alternating current power consume both real power, which does or is able to do useful work, and reactive power, which dissipates no energy in the load and which returns to the source on each alternating current cycle. The vector sum of real and reactive power is the apparent power. The ratio of real power to apparent power is the power factor, a number between 0 and 1 inclusive. The presence of reactive power causes the real power to be less than the apparent power, and so, the electric load has a power factor of less than unity.
The reactive power increases the current flowing between the power source and the load, which increases the power losses through transmission and distribution lines. This results in additional costs for power companies. Therefore, power companies require their customers, especially those with large loads, to maintain their power factors above a specified amount (usually 0.90 or higher) or be subject to additional charges. Electricity utilities measure reactive power used by high demand customers and charge higher rates accordingly. Some consumers install power factor correction schemes at their factories to cut down on these higher costs.
Electrical engineers involved with the generation, transmission, distribution and consumption of electrical power have an interest in the power factor of loads because power factors affect efficiencies and costs for both the electrical power industry and the consumers. In addition to the increased operating costs, reactive power can require the use of wiring, switches, circuit breakers, transformers and transmission lines with higher current capacities.
Power factor correction brings the power factor of an AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors which act to cancel the inductive or capacitive effects of the load, respectively. For example, the inductive effect of motor loads may be offset by locally connected capacitors. Sometimes, when the power factor is leading due to capacitive loading, inductors are used to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth between each AC cycle.
Instead of using a capacitor, it is possible to use an unloaded synchronous motor. The reactive power drawn by the synchronous motor is a function of its field excitation. This is referred to as a synchronous condenser. Such a condenser is started and connected to the electrical network. It operates at full leading power factor and puts reactive power (commonly referred to as Volt-Amps Reactive or “VARs”) onto the network as required to support a voltage of a system or to maintain the system power factor at a specified level. The installation and operation of a condenser are identical to large electric motors. Its principal advantage is the ease with which the amount of correction can be adjusted, as it behaves like an electrically variable capacitor.
Non-linear loads create harmonic currents in addition to the original AC current. Addition of linear components such as capacitors and inductors cannot cancel these harmonic currents, so other methods such as filters or active power factor correction are required to smooth out their current demand over each cycle of alternating current and so reduce the generated harmonic currents.
A typical switched-mode power supply first rectifies a AC current, forming a DC bus (or DC ripple current) using a bridge rectifier or similar circuit. The output voltage is then derived from this DC bus. The problem with this is that the rectifier is a non-linear device, so the input current is highly non-linear. That means that the input current has energy at harmonics of the frequency of the voltage.
This presents a particular problem for the power companies, because they cannot compensate for the harmonic current by adding simple capacitors or inductors, as they could for the reactive power drawn by a linear load. Many jurisdictions are beginning to legally require power factor correction for all power supplies above a certain power level.
FIG. 1 illustrates the current and voltage waveforms for an electronic device that power factor correction (PFC) is designed to correct according to the prior art. As illustrated, the voltage waveform is sinusoidal in shape and the current waveform can be characterized as a waveform with a steady current value with large spikes in the amplitude of the current waveform along with a high content of harmonics. The large spikes in the current waveform are caused because of the switching power supply and its use of the rectifier bridge/smoothing capacitor circuits. From an efficiency viewpoint, a typical uncorrected switched-mode power supply has a power factor of 0.6, which effectively reduces the current available from the AC socket from about 13 to about 7.8 Amps.
A solution for power factor correction is to condition the equipment's input load power so that it appears purely resistive using active PFC techniques. Common PFC designs employ a boost preconverter ahead of the conventional voltage-regulation stage, which effectively cascades to switched-mode power supplies. The boost preconverter raises the full-wave rectified, unfiltered AC line to a DC input rail at a level slightly above the rectified AC line, can be around 375 to 400 volts DC. By drawing current throughout the AC line cycle, the boost preconverter forces the load to draw current in phase with AC line voltage, quashing harmonic emissions.
The simplest way to control the harmonic current is to use a filter as a passive power factor correction technique. It is possible to design a filter that passes current only at line frequency (e.g., 50 or 60 Hz). This filter reduces the harmonic current, which means that the non-linear device now looks like a linear load. At this point the power factor can be brought to near unity, using capacitors or inductors as required. This filter requires large-value high-current inductors, however, which are bulky and expensive. This is a simple way of correcting the nonlinearity of a load by using capacitor banks. It is not as effective as active PFC. Switching the capacitors into or out of the circuit causes harmonics, which is why active PFC or a synchronous motor is preferred.
It is also possible to perform active power factor correction. For such, a boost converter is commonly inserted between the bridge rectifier and the main input capacitors. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switch mode converter inside the power supply produces the desired output voltage from the DC bus. This approach requires additional semiconductor switches and control electronics, but permits cheaper and smaller passive components. Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power from about 100 V (Japan) to 240 V (UK).
An Active Power Factor Corrector (active PFC) is a power electronic system that controls the amount of power drawn by a load in order to obtain a Power Factor value as close as possible to unity. In most applications, the active PFC controls the input current of the load so that the current waveform is proportional to the mains voltage waveform (a sine wave). Some types of active PFC are (i) Boost, (ii) Buck, and (iii) Buck-Boost Active power factor correctors can be single-stage or multi-stage. Active PFC can produce a PFC of 0.99 (99%).
Power supplies that utilize rectifier-bridge/smoothing capacitor circuits draw non-sinusoidal currents as the instantaneous voltage of the AC line exceeds the voltage of the storage capacitor. The electricity generator, with no power factor correction, must supply energy at the top/peak of the sine wave rather than throughout the cycle, which can cause the sine wave to collapse around its peak.
FIG. 2 illustrates a power factor correction circuit with a boost preconverter according to the prior art. The full-wave bridge rectifier 200 receives the AC input voltage and produces a full-wave rectified voltage. The boost preconverter 205 receives the full-wave rectified voltage and forces the load to draw current in phase with the voltage. The shape of the current waveform is determined by a switching device 215, which is coupled to the output and a control circuit 220. The control circuit 220 provides an input to the switching device 215 and receives as input signals a signal from the output and a signal from the rectifier/boost node 225. This circuit may solve the power factor problem by shaping the current waveform to mimic the voltage waveform and to cause the current waveform to be in phase with the voltage waveform.
For some applications, including those providing power at relatively high voltages, such previously described PFC techniques can present or allow for undesirable losses in efficiency due to non unity PFC values.
Increasingly, many industrial, commercial, and public infrastructure applications have utilized light emitting diodes for lighting. Compared with previous lighting techniques such as incandescent or fluorescent lighting, LEDs can provide, a broad color spectrum, compact size, increased energy efficiency, absence of mercury and related environmental concerns, increased operating life, ability to dim output, absence of infrared or ultraviolet spectral components (when desired), and low voltage (on a per LED basis). LEDs are inherently low voltage devices and depending on color and current, the forward voltage of the LED can vary from less than 2 to 4.5 V. In addition, LEDs need to be driven with a constant current to ensure the intensity and color desired. Regarding driver stages for electrical components such as various types of lighting, including LEDs, regulators have been used for power regulation and power factor correction. Such regulators and PFC techniques, however, have been shown to have less than optimal current control. This in turn can lead to unacceptable variation in current delivery, with attendant component longevity reductions and thermal management issues.
What is currently lacking, therefore, are techniques for providing power factor correction values closer to unity under a variety of operating condition and for relatively high voltages. What is further lacking are techniques for providing relatively high voltages for electrical components with increased and more uniform power regulation, particularly for lighting applications, including LED applications, where variations in applied power can produce noticeable visual effects.